The present invention relates generally to computers, and more particularly to communications between interconnected computer nodes, storage subsystems, and other network devices.
Assessments of server and I/O technologies and their respective marketplaces make clear that cluster communications, including server-to-server communications and server-to-I/O communications, will be increasingly based on a distributed model. Existing server architectures, based on a shared-address-space model, bus-oriented connections to I/O devices and I/O transactions based on a load/store memory model, have limitations.
FIG. 1 illustrates a block diagram of a current generation computer, including CPUs 105 and 110 connected to a host bus 111, CPU 105 is connected to a cache 106 and CPU 110 is connected to a cache 108. The system also includes main memory 109, and one or two host/PCI bridges 112 and 115. Host/PCI bridges 112, 115 convert transactions between host bus 111 and a Peripheral Component Interconnect (PCI) bus 113. A storage controller 324 and several I/O devices 120, 122 and 124 are connected to PCI bus 113. I/O devices 326 are also connected to storage controller 324.
There are limitations on the number of electrical loads which may be placed on the host bus 111. Moreover, these configurations are also limited by the PCI bus 113, which imposes limits both on the number of electrical loads it is capable of supporting and the physical distances at which devices can be placed. As illustrated in FIG. 1, storage connectivity and proximity are typically restricted to what fits within a single enclosure.
An architecture for I/O pass through of the present invention overcomes some of the disadvantages and limitations of prior art computer systems by increasing the number of I/O devices that can be connected to a computer system, by increasing the distance at which the I/O devices are located, and by permitting a more distributed architecture. The distributed computer system of the present invention increases the number of I/O devices and the distance at which they are coupled to host computers, while retaining some of the features of current I/O buses, such as PCI. However, current packet formats are impractical for usage in such a distributed computer system of the present invention. For example, the packet size used in Asynchronous Transfer Mode (ATM) is 53 bytes. However, 53 bytes is very unfriendly for use in a 32 byte cache line. Moreover, current packet formats cannot be easily used to transport host and I/O transactions without requiring significant circuitry for decoding commands or opcodes.
As a result, a need exists for an improved packet format that is more compatible with the distributed computer system of the present invention.